The recent study (Rad. Res. 65:83-97, 1976) demonstrates that an acute x-ray dose of 1.0 R can induce erythroid changes in the bone marrow of the rats. The change observed between 10 days and 10 weeks following exposure, manifested as an increased level of normoblasts measured as a per cent of nucleated marrow cells in stained marrow smears, and as the number of normoblasts per mgm of isolate marrow, per whole femur and per total skeleton. When assayed at six or ten weeks post irradiation under induced anemic conditions (81 hours post phlebotomy) the fraction of increase in the number of normoblasts in the skeletons of the exposed subjects was only approximately one-half of its normal level. It was further observed that the reduced capacity of erythroid response was repaired at a rate of approximately 50% per 30 weeks. Because this is the dose level of radiation that urgently needs to be critically analyzed for health hazard evaluation, and because this assay can be made in a very critical organ with relatively few animals, this in vivo marrow model would be ideal for the study of the effects of low dose radiation. It is proposed to employ this model (1) to determine the lowest dose of an acute and the lowest dose-rate of a continuous irradiation capable of inducing a biological effect, (2) to characterize the dose-response relationship in animals exposed to radiation dosages in the range of 1.0 R and lower, (3) to examine if a dose of radiation too low to elicit a detectable response can, when given repeatedly, ultimately induce a measurable change and (4) to determine the rate of recovery of the deficit erythroid response induced by acute irradiation. The results obtained will yield information that will be of direct use (without extrapolation) in the determination of the exposure limitations of low dose radiation.